This invention relates to the field of video projection display, and in particular to the measurement of projected image illumination of a photo sensor.
In a projection video display, geometrical raster distortions result from the physical placement of the cathode ray display tubes. Such raster distortions are exacerbated by the use of cathode ray tubes with curved, concave display surfaces and the inherent magnification in the optical projection path. The projected image is composed of three scanning rasters which are required to be in register one with the other on a viewing screen. The precise overlay of the three projected images requires the adjustment of multiple waveforms to compensate for geometrical distortion and facilitate the superimposition of the three projected images. However, manual alignment of multiple waveforms is labor intensive during manufacturing, and without the use of sophisticated test equipment may preclude setup at a user location. Thus an automated convergence system is disclosed which simplifies manufacturing alignment and facilitates user location adjustment. An automated alignment system may employ raster edge measurement at peripheral screen locations in order to determine raster size and convergence, however, wide variations in sensor illumination levels may result in inaccurate measurement. To obviate such measurement inconsistencies it is necessary compensate for the variations in sensor illumination.
A method of operating a photo sensor in a projection display device comprises the steps of, illuminating the sensor with a projected image. Detecting a signal from the sensor responsive to the illumination by the projected image. Changing a threshold value of the detector until the signal detection ceases. Storing a value representative of the threshold value which terminated detection of the signal from the sensor.